This has been irritating me for quite some time, Gimp has a lot of great functionalities but if it can't manage this very basic capability then I'll have to do settle with Pinta.
Suppose We want to Fill an area in this image
The bucket tool uses the foreground colour so lets change it-- Click foreground
Set new colour
Click OK to accept
Foreground colour now changed select bucket tool
Use bucket tool -- Job Done
The black square in the highlighted area defines the foreground color. This color applies to all coloring tools, like the bucket